Today, the Arctic is synonymous with "cold." But a new study suggests the polar region has experienced periods of intense warmth over the past 2.8 million years that may have been hot enough to melt the Greenland ice sheet.
Scientists already knew that the Arctic is warming twice as fast as the global average. But the new study, based on a sediment core drilled from a Russian lake, suggests the far north's climate is even more sensitive than researchers suspected.
"There are really big surprises in the history of the Arctic that are revealed in the cores," said Julie Brigham-Grette, a geologist who studies the climate of the past 3 million years.
She's a co-author of the study and was the lead American scientist on the international project to drill the sediment core at Lake El'gygytgyn in the northeastern Russian Arctic. Its results were published yesterday in the journal Science.
"We've identified almost a dozen intervals in the core over the last 2.8 million years when the climate was naturally much warmer than the present," Brigham-Grette said. "It gives us a very interesting clue about how sensitive the Arctic is to change."
Conditions were so toasty during two of the warm periods, between 400,000 and 1.1 million years ago, that scientists believe the Greenland ice sheet may have disappeared entirely.
The question now for researchers is why.
Increasing concentrations of greenhouse gases in the atmosphere have driven recent warming of the Earth's climate. And wobbles in the planet's rotation probably kick-started the end of the last ice age, about 19,000 years ago, by bringing Earth closer to the heat of the sun.
But what scientists know about ancient greenhouse gas levels and changes in Earth's rotation doesn't fully explain why the Arctic was so much warmer millions of years ago, at the farthest reaches of the period documented by the new lake core, according to climate modeling results included in the new study.
Previous research on carbon dioxide levels in the atmosphere, for example, suggests they were not as high then as they are today.
Causes of earlier warmth not clear
Brigham-Grette and her colleagues believe that the missing ingredient may be feedback loops that acted to intensify warming started by other factors, especially in the Arctic.
They are likely the same feedback processes scientists are seeing evidence of now and projecting for the near future, she said -- such as shrinking Arctic sea ice driven by rising temperatures.
As the Arctic's icy cap recedes, it enlarges the area covered by dark, open water that absorbs more heat than the white ice it replaces. That creates a circle that hastens the melting of the ice that remains.
There are other tantalizing clues buried in the Lake El'gygytgyn sediment core.
The scientists noticed that many of the dozen warm periods in the sediment record over the last 2.8 million years seem to have occurred when the West Antarctic ice sheet melted away, as documented in Antarctic sediment cores.
They speculate that the warm periods at the poles may be connected -- that those warm periods intensified first in Antarctica, melting the West Antarctic ice sheet.
That might have altered ocean circulation, warming surface water in the north Pacific Ocean and eventually temperatures throughout the Arctic.
Antarctic melt may have accelerated the Arctic's
Or the loss of West Antarctica's ice could have raised the sea level, pushing warm surface water through the Bering Strait into the Arctic Ocean and warming the region.
For researchers trying to understand the future climate, studying the distant past is important because it can reveal climate surprises -- forces that shaped the ancient climate that may crop up again.